1. Field of the Invention
The present invention relates to optical magnetic field sensors utilizing a magnetooptical effect of magnetic garnet materials. In particular, the present invention relates to optical magnetic field sensors which have high sensitivity and excellent magnetooptical characteristics and which are to be used for detecting accidents such as earthing in power distribution lines.
2. Related Art Statement
Electrical type measuring instruments have conventionally been used as measurement instruments for controlling electric current and voltage in an electric power field using high voltage and high electric current. For this purpose, the measuring instruments must be of a very large size and have high insulation reliability so that the measuring instruments themselves may not be a cause for earthing or shorting accidents. Thus, there are problems in that the cost is high and a large installation area is necessary.
In order to solve the problems of the above-mentioned measurement instruments, optical magnetic field sensors in which a Faraday element having a magnetooptical effect (for example, a magnetic garnet material) is combined with an optical fiber are disclosed in K. KYUWA et al., IEEE Journal of Quantum Electronics, Vol. QE-18, No. 10, 1619 (1982)).
The optical magnetic field sensor of this type comprises a light source section composed of a light emitting diode or the like, a magnetic field-detecting section constituted by interposing a Faraday element made of, for example, a magnetic garnet material, between a polarizer and an analyzer, a measurement circuit for receiving and measuring light emitted from the light source section and passing through the magnetic field-detecting section, and light transmission paths using optical fibers for optically connecting these sections. Owing to this construction, since the magnetic field-detection section is merely optically connected with the measuring circuit by the optical fiber, but electrically insulated therefrom, insulating reliability is high, and since the light transmission section is constituted by the optical fibers, they do not undergo electromagnetic induction so that a magnetic field detector having a sufficient measuring precision can be installed at a low cost by simple means.
The above magnetic garnet material is known to have a particularly great magnetic field-detecting sensitivity (Verdet constant) among Faraday elements such as Faraday glass and bismuth silicon oxide. Therefore, in order to improve the characteristics of the optical magnetic field sensors, various contrivances have been made on the magnetic garnet material having the Faraday effect.
For example, Japanese patent application Laid-open Nos. 62-150,185 and 62-188,982 propose that the magnetic field can be measured at high precision by specifying the orientations of planes of the magnetic garnet single crystal material. Further, Japanese patent application Laid-open No. 63-163,815 proposes that a polycrystal of magnetic garnet is made transparent by adding calcium thereto to make it possible to measure the magnetic field at high precision.
However, an FZ process, an LPE process, a flux process or the like are generally used as production processes for producing the single crystal materials of the above-mentioned magnetic garnet. However, since these processes grow single crystals from a melt of garnet, they suffer problems in that specialized equipment is necessary, mass productivity is poor, and variations in the characteristics of the materials are likely to occur.
Although the magnetic garnet polycrystal which is made transparent by adding calcium as mentioned above has a better mass productivity than the above-mentioned single crystal materials, the polycrystal has the following problems. That is, it first suffers the problem that when calcium is added, sensitivity of the sensor drops because the magnetooptical characteristics, i.e., the Faraday rotation angle and the Verdet constant become smaller. Next, since vanadium needs to be added to supplement charges of calcium, the kinds of elements to be mixed during the production becomes greater Consequently, an inner portion of a sintered body is likely to become inhomogeneous so that variations of the magnetooptical characteristics of the sensor are likely to occur.
When these magnetic garnet materials are used as optical magnetic field sensors for the power distribution, the sensitivity of the magnetooptical element, uniformity of the element and its output stability are sometimes inadqueate. Earthing accidents in the power distribution are detected by arranging three optical magnetic field sensors in three phase power distribution line, uniting their detected magnetic fields, and detecting about 100 mA earthing current in the case of a load current being 600 A. Therefore, a detecting precision of not more than 1/10,000 is required.
In order to reduce the weight of the sensors in the power distribution lines, structures without iron cores are desired. Particularly, magnetic garnet materials having high sensitivity is demanded.